Simultaneous digital transmission in both directions over one line

ABSTRACT

A duplex digital signalling system includes a transmission line having characteristic impedance terminations, a transmitter at each end of the transmission line for supplying digital signals through the characteristic impedance termination to the transmission line, and a differential receiver at each end of the transmission line. A resistor network couples each receiver across the local characteristic impedance termination, so that each receiver responds only to the distant transmitter, and digital information may be simultaneously transmitted in both directions through the transmission line.

iififi States Patent 91 Davis Apr. 3, 1973 William John Davis, NorthPalm Inventor:

Primary ExaminerJohn W. Caldwell Assistant Examiner-Marshall M. CurtisAtt0rney-H. Christoffersen and Carl V. Olson B h Fl [57] ABSTRACT eac a.A duplex digital signalling system includes a transmis- [73] Asslgnee:RCA corporationNew sion line having characteristic impedance termina- 22Filed; 9 1970 tions, a transmitter at each end of the transmission linefor supplying digital signals through the charac- [21] PP teristicimpedance termination to the transmission line, and a differentialreceiver at each end of the 52 U.S.Cl ..l78/58 178/60 transmlssi line- Aresistor network Couples each [51] Int. Cl. ..Hl)4l 5/14 receiver acrossthe local characteristic impedance [58] Field of Search l78/58 60mination, so that each receiver responds only to the distanttransmitter, and digital information may be [56] References Citedsimultaneously transmitted in both directions through the transmissionline.

UNITED STATES PATENTS 3 Claims, 4 Drawing Figures 3,573,371 4/197]Carbone et al. ..l78/60 K f a I A I 1 fH'i Vif Fifi/Vi i a i [e1 I? ,L I6 'f/fl I: ,4" I g Ki 2K f" 24 f,

I re K /K A Z 1K j rm/w- W /7 A 7561/1 5- f luff/775A A A?! a Z Z a if]4 5 fl C 1 W I 7 f? 2 i2 5 BACKGROUND OF THE INVENTION Computer systemsoften involve a plurality of units, such as basic processors, memoriesand input-output devices, which are located some distance apart and areconnected by cables. Since a considerable number of signal lines must beprovided, the cost, complexity and space requirements of the cablingsystem are substantial.

SUMMARY OF THE INVENTION In order to reduce the number of signal cables,a system is provided which permits the simultaneous transmission in bothdirections of digital signals over a single line. The line may be aconductor pair, or a single conductor with a common ground return. Theline conductors are provided with characteristic impedance terminatingresistors, and a differential receiver at each end of the line iscoupled by a resistor network to both sides of the local terminatingresistor. The network is constructed so that a receiver is responsive toa distant transmitter, and is unresponsive to a local transmitter.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a diagram of a balancedtransmission line having send-receive units at each end which permitdigital information to be simultaneously transmitted in both directionsthrough the transmission line;

FIG. 2 is another embodiment of the system of FIG. 1 which differs inthat it utilizes unipolar signals instead of bipolar signals;

FIG. 3 is another embodiment of the invention employing a single-endedtransmission line and bipolar signals; and

FIG. 4 is another embodiment of the system of FIG. 3 using unipolarsignals instead of bipolar signals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made toFIG. 1 for an illustration of a duplex digital signalling systememploying bipolar voltage signals, and including a balanced transmissionline L and two transmit-receive units TR and T'R'. The transmission lineL consists of two conductors L and L having a characteristic impedance Zand having any desired length up to a length at which the directcurrentresistance of the line is significant in relation to the resistance ofthe terminating resistors. Lengths up to 200 feet and greater areentirely practical. The transmission line may, for example, have acharacteristic impedance of about 140 ohms. In the transmit-receive unitTR, the transmission line conductors L, and L are provided withseries-connected, characteristic impedance terminating resistors R eachhaving a value equal to one-half the balanced characteristic impedanceof the line.

5 The transmit-receive unit TR includes a transmitter T having outputsconnected to the terminals A and B of the terminating resistors R,, andhas a logic input terminal I for receiving 1" and 0 digital informationsignalsto be transmitted.

The transmit-receive unit TR also includes a differential receiver Rhaving a normal or positive input terminal E, and an inverting or anegative input terminal F. The input terminals E and F of the receiverare connected by a resistor network to terminals A, B, C and D of theterminating resistors R,. The resistor network includes a resistor Rconnected between transmitter output terminal B and receiver inputterminal E, a resistor R,,, connected between transmitter outputterminal A and receiver input terminal F, a resistor R connected fromthe line terminal C to the receiver input terminal E, and a resistor Rconnected from the line terminal D to the receiver input terminal F. Theresistors R and R are preferably of equal value and may have a value of2,000 ohms, by way of example. The resistors R and R are preferably ofequal value and have a value half that of the previously recitedresistors, so that they are, in the example, equal to 1,000 ohms each.

The transmitter T may, for example, be any suitable integrated circuitunit commonly known as a differential line driver having a bipolaroutput. The Type 9614 differential line driver, made and sold by Fairchild Semiconductor Division of Fairchild Camera 35 and InstrumentCorp., may be used with the addition of an output stage providing abipolar output. The differential receiver R may, for example, be anysuitable commercially-available integrated circuit unit such as the TypeSN75107 line receiver manufactured and sold by Texas Instruments, Inc.

The transmit-receive unit T'R' at the opposite end of the transmissionline is identical with the send-receive unit TR and is shown as a mirrorimage in the drawing 45 with the corresponding elements having the samedesignation with a prime mark added.

In the operation of the system of FIG. 1, there are four differentdirect-current conditions possible in the system depending on thedigital information bits being simultaneously transmitted bytransmitters T and T. That is, transmitter T may be transmitting a 1" ora 0. When transmitter T is transmitting a 0," transmitter T may betransmitting either a 0 or a 1. Similarly, when transmitter T istransmitting a l,

transmitter T may be transmitting either a 0 or a 1. Therefore, thereare four possible direct-current conditions in the system as shown inthe following chart:

TABLE 1.CONDIIIONS IN FIG. 1

The first line in the above table represents the conditions in thesystem when the logic inputs to both transmitters T and T are logicdigital signals. In this case, the voltages at the outputs A and B oftransmitter range of :4 volts or $4,000 mV, and the receiver may have athreshold uncertainty range of only 125 mV. Allowing for the voltagedividing action in the resistor network, the receiver thresholduncertainty voltage T are e and +e, respectively. Similarly, thevoltages at 5 range is only about 1.25 percent of the transmitter outtheoutputs A and B of transmitter T are also e and put voltage swing. Atransmitter voltage pulse rise time +e, respectively. Since the systemis perfectly symmetor fall time may be l0 nanoseconds, and 100 feet ofrical, and since the transmission line is terminated at cable may have arise or fall time of 50 nanoseconds. both ends in its characteristicimpedance, the voltage Then, t time during which thfl 4,000 mv changc -eexists at points A and C, on the line d t r L passes through the :25 mVthreshold uncertainty of a d at points C d A' Si il -1 h voltage existsreceiver is only about 1.25 nanoseconds. The receiver at points B d D,on h li conductor L and at cannot respond in such a short time toproduce an inpoints D dB', correct logic output. The timing of thecorrect logic The voltage at receiver input E is determined by theOutput of the receiver y be displaced smah amount voltage dividingaction of resistor R connected to +e th the tahge of about :2nanoseconds: This is t h' volts at point B, d resistor Ree connected tovolts cant ln relatlon to nor mal response tlme varlatlons 1n at pointC. The voltage at receiver input E is thus e/3. the system h to Chou"and cable delays The voltage at receiver input terminal F is determinedThe deschbed h 1t advahtage that the by the voltage dividing action ofresistor R,,, connected 231? r g li e g gi dg l 0 22 6 ha l; zsg ggg wits giz to e volts at point A and resistor R connected to +e volts atpoint D. The differential l le voltage EF apnumber h h two separate Wplied to receiver R is thus equal to -2/3e when the 215383 322332 2532523 22 3: t ggs i f l input terminal E is considered as the referencepoint. The receiver R responds to the negative differential 25 z cz gzgll glf igggga ggjz gfgi iz 3 553 32112 u n a Input i k f tolpmduce aloglc 0 output on its wise have when used i'ith a one direction line Theoutput ine his ogic output is in response to the0 receivers thereforeare somewhat less immune to noise loglc Input Sllpphed to mput termmalI, of the remote disturbances on the line Nevertheless the describedtransmitter T l s stem can be constructed to rovide its intended o illiiififli iol oliil iiie'liii iiiiliii? i eyconomic and Practicalstages and be com letel reliable in the transmission of di ital infor-Fecewer R or R responds to a negatwe dlfferennal mati n in bothdirections on the transmission line. lnput voltage EF or E'F' to producea 0 logic output,

. I Reference ls now made to FIG. 2 where there 15 and responds to aposltlve dlfferentlal lnput voltage to Shown a System which is the Sameas Shown in FIG 1 I Q l I 4 produce? 1 .loglc l H F aloglcmputlt? exceptthat the electrical signals produced at transa transmitter is a loglc 0,the loglc output 0 or 0 f th t l l 0 l mltter output termlnals A and Bare unipolar slgnals ln fi f T f 1 Ogle which a logic 0 is representedby 0 volts at A and +e I a h e (iglc ga 0 h o a ransml er Ogle volts atB, and a logic 1 is represented by +e at A 5 9, put or o t e remoterecelvfl a ways 40 volts and 0 volts at B. The system of FIG. 2 includesada Ogle ditional resistors R R R' and R each connected 1 r Theforegthhg Thble 1 describes the dhect h' from a receiver input terminalto a point of zero or voltage condltlonsln the system to explaln howlnforground potential. These resistors are preferably of the matlon canbe simultaneously transmitted ln both Same value and in the example, mayhave a value f dlrectlons over the transmlsslon llne L. The system also5 2,000 ohms. P t to Correctly t t In both The transmitter T may, forexample, be any suitable dhectlohs hhdet dyhamw cohdltlohsv regal'dlfissof the commercially available integrated circuit unit comthhlhg ofstghals PP to the hhe from both trans monly known as a differential linedriver, such as the mlttfits T and when the two ttahsthtttets pf ih Type9614 differential line driver made and sold by a completely asynchronousmanner, It is Possible for Fairchild Semiconductor Division of FairchildCamera the local transmitter output to change state immediatedInstrument C Th differ ntial receiver R may, 1y pr ceding, ex c lycolncldcnt Wlth, r sllghtly after for example, be any suitablecommercially available inan incomlng wavefront is recelved that waspropagated tegrated ir it nit su h as the Type SN75107 line from theremote transmitter. However, the transmitter receiver n f ctured andsold by Texas Instruments, voltage swings are very large and very rapidin passing In through the receiver threshold voltage range in which Theoperation of the system of FIG. 2 is similar to the the receiver isuncertain in producing a 1 or a 0 operation of the system of FIG. 1. InFIG. 2, the condilogic output. tions existing under the four possiblesignalling condi- I The transmitter output voltage may change over ations are as shown in the following table:

TABLE 2.CONDITIONS IN FIG. 2

TR TR [ll Voltage Out Out Voltage IN l A l! o I) EF 0 o EF D c B A I 0"u l- 0 o -v-/ l 0" 0" e/4 o o o 0 0" 1" lt) 2 or. '('/'l 11" 1" +(l/4 oz o z o 0 0" 0" O 1 9/2 l'/.2 +l /l 0" 12/4 P/Z (/2 f) t 1" l 1'. (l l.l) +l'/l "l" 1" +e/4 U (I. U l 1" A receiver R or R produces a logic 0output signal when the differential input signal EF or EF' is a negativevoltage, and produces a logic 1" output signal when the differentialinput signal is a positive voltage.

Reference is now made to FIG. 3 for an illustration of anotherembodiment of the invention for use with a single-ended transmissionline having a single conductor and a common ground or return path. Thevoltages supplied by the transmitters are bipolar signals in which alogic 0 is represented by -e volts at terminal A or A, and a logic 1 isrepresented by a +e volts at terminal A or A. The system of FIG. 3includes a single terminating impedance R at each end of the singleendedconductor. Each terminating impedance has a value equal to thecharacteristic impedance of the line. In operation, the four possibleconditions of the system are as listed in the following table:

'Tim 3.CONDITIONS IN FIG. 3

Reference is now made to FIG. 4 for an illustration of an embodiment ofthe invention including a singleended transmission line, and beingdifferent from the embodiment of FIG. 3 in that the signals applied fromthe transmitters to the line are unipolar. That is, the signal appliedby a transmitter to point A or A is 0 volts to represent a logic 0," andis +e volts to represent a logic 1 The system of FIG. 4 differs from thesystem of FIG. 3 in that the resistors connected from the inputterminalsof the receivers are connected to a source of bias potentialequal to +e/2, rather than to a point of 0 or ground potential. The fourdifferent conditions of the system are as listed in the following table:

TABLE 4.CONDITIONS IN FIG. 4

The four systems shown in FIGS. 1 through 4 include resistor networkshaving relative resistor values which provide a maximum signal voltageswing into the receivers, while maintaining input voltage magnitudes atthe same positive values for the two inputs producing a 1" output, andat equal negative values for the two inputs producing a 0 output. Otherresistor values may be used, but they will involve a reduction in noiseimmunity, and/or an increased variation in the time responses of thereceiver.

Other transmitter-receiver unit configurations may be employed in whichthe resistor networks are constructed to maximize the differential inputsignals at the receiver input terminals, and provide differential inputsignals of equal magnitudes for all logic conditions.

What is claimed is:

1. The combination of a balanced transmission line,

a transmit-receive unit at each end of the transmission line, eachtransmit-receive unit including:

a characteristic impedance termination including a terminating resistorin circuit with each conductor of the transmission line,

a transmitter having first and second output terminals for supplyingdigital voltage signals through the terminating resistors to thetransmission line,

a differential receiver having a normal input terminal and an invertinginput terminal,

first and second resistors of substantially equal values connected fromthe ends of said transmission line to respective normal and invertinginputs of said receiver,

a third resistor of value larger than said first and second resistorsconnected from the first output of said transmitter to the invertinginput of said receiver, and

a fourth resistor of value equal to said third resistor connected fromthe second output of said transmitter to the normal input of saidreceiver.

2. The combination as defined in claim 1 and in addition, fifth andsixth resistors connecting the inputs of the receiver to a point ofreference potential.

3. The combination as defined in claim 1 wherein said third and fourthresistors have a value equal to substantially twice the value of saidfirst and second resistors.

1. The combination of a balanced transmission line, a transmit-receiveunit at each end of the transmission line, each transmit-receive unitincluding: a characteristic impedance termination including aterminating resistor in circuit with each conductor of the transmissionline, a transmitter having first and second output terminals forsupplying digital voltage signals through the terminating resistors tothe transmission line, a differential receiver having a normal inputterminal and an inverting input terminal, first and second resistors ofsubstantially equal values connected from the ends of said transmissionline to respective normal and inverting inputs of said receiver, a thirdresistor of value larger than said first and second resistors connectedfrom the first output of said transmitter to the inverting input of saidreceiver, and a fourth resistor of value equal to said third resistorconnected from the second output of said transmitter to the normal inputof said receiver.
 2. The combination as defined in claim 1 and inaddition, fifth and sixth resistors connecting the inputs of thereceiver to a point of reference potential.
 3. The combination asdefined in claim 1 wherein said third and fourth resistors have a valueequal to substantially twice the value of said first and secondresistors.